The reverberation chamber, or mode-stirred chamber, was before year 2000 only known as an instrument to measure radiated emissions and susceptibility to radiation, i.e. for electromagnetic compatibility (EMC) testing. The required measurement uncertainty was then not so strict.
The U.S. Pat. No. 7,444,264 describes how the reverberation chamber could be used to measure e.g. the radiation efficiency of antennas and the total radiated power (TRP) of mobile and wireless terminals such as cellular phones. Several chamber improvements were introduced to get the desired uncertainty, which is much stricter than the EMC case.
The same measurement setups that are described in U.S. Pat. No. 7,444,264 were also used to determine the performance of antenna diversity, i.e. when the outputs of two antennas are combined in such a way that the deepest fading dips are reduced, see e.g. P.-S. Kildal and K. Rosengren, “Correlation and capacity of MIMO systems and mutual coupling, radiation efficiency and diversity gain of their antennas: Simulations and measurements in reverberation chamber”, in IEEE Communications Magazine, Vol. 42, No. 12, December 2004.
The above-mentioned first works describe how antennas and transmitting mobile and wireless terminals, intended for use in fading multipath environment, can be characterized by measurements in a reverberation chamber. However, there was also a need for characterizing mobile and wireless terminals when they are receiving.
The receive performance is either characterized by a Bit-Error-Rate (BER) or a Frame-Error-Rate (FER), depending on which system the terminals are designed for, where the latter frame consist of several bits that are coded in a special way to reduce errors. The BER or FER will depend on the signal level present at the receiver. Therefore, the receiver sensitivity is defined as the level which provides a certain BER or FER, often chosen to be 0.5%. It is known how to measure the receiver sensitivity when a signal is connected directly to the port of the receiver of the terminal. This is often referred to as conductive measurements because the transmit signal is connected directly to the receiver without including any antenna or environment. Then, however, the performance of the antenna is not included in the measurements. Therefore, it has previously been described how to measure the receiver sensitivity in an anechoic chamber. This is done by using a base station emulator connected to the transmit antenna in the chamber, and locating the terminal on a turntable. The receiver sensitivity for a certain BER or FER is then determined by analyzing the received signal at the phone, at each of all the directions of incidence on the terminal. The latter directions are obtained by moving the turntable in the anechoic chamber. These receiver sensitivities will vary much with direction, because the received radiation pattern of the terminal is different for the different directions. Therefore, these values are averaged over all directions (which should be uniformly distributed over the complete unit sphere around the terminal). The averaged results are called Total Isotropic Sensitivity (TIS), and correspond to the conductive-measured receiver sensitivity minus the total radiation efficiency of the antenna. This TIS can also be measured in a reverberation chamber, by averaging over mode stirred positions and polarizations, thereby corresponding to the measurements of radiation efficiency when the terminal is receiving.
The above-mentioned procedure for measuring TIS is very time-consuming, both when it is performed in an anechoic chamber and in a reverberation chamber, because the sensitivity must be determined many times and averaged. Also, it does not test how the terminal works when it is exposed to a continuous fading of the input signal, which is representative of an actual environment.
Therefore, it was the purpose of U.S. Pat. No. 7,286,961 to describe how the reverberation chamber can be used to determine the receive performance of a mobile or wireless terminal when it is continuously exposed to a fading input signal, such as in a real environment. The continuous fading is obtained by moving the stirrers of the chamber continuously rather than in steps, and by measuring the receiver sensitivity for which the BER or FER has the desired value during this continuous movement of the stirrers. The latter method is called Average Fading Sensitivity (AFS) and is much faster than measuring TIS, because the sensitivity level only needs to be determined once.
The reverberation chamber can by making use of the above previous inventions be used for characterizing the complete performance of mobile and wireless terminals, both on transmit and receive, including transmit and receive performance of antennas, amplifiers, signal processing algorithms, and coding. This has opened up a large potential for RF testing in connection with terminals for more advanced future mobile communication systems referred to as 3G and 4G (third and fourth generation of mobile communication systems, also called LTE). Such systems make use of more than one antenna for both transmission and reception and will use these to adapt to the fading multipath environment, in order to improve battery life time and data rate. Such systems are known under terms as diversity antenna systems and MIMO (multiple Input Multiple Output) antenna systems. In order to develop optimum diversity and MIMO systems it will be more important than ever to quantify the performance of the terminals and base station simulators in multipath environments. The reverberation chamber can provide this testing opportunity.
The ultimate testing opportunity of the reverberation chamber is to measure data throughput of the whole communication system with diversity and MIMO capability, from the data input at the base station to the data output at the terminal, or vice versa. This contains the effects of radiated power, the wireless channel and receiver sensitivity in one performance value, referred to as the throughput, being the most important for the user. This throughput is a resulting data transfer rate, and the measurement setup in reverberation chamber is already described in scientific paper (J. Åsberg, A. Skårbratt, and C. Orlenius, “Over-the-air performance testing of wire-less terminals by data throughput measurements in reverberation chamber”, European Conference on Antennas and Propagation ICAP 2011, 11-15 Apr. 2011, Rome).
The reverberation chamber can be loaded by using lossy objects inside the chamber or lossy material on the reflecting walls, in order to control the coherence bandwidth and time delay spread so that it resembles values in real-life environments. This loading affects the average mode bandwidth of the chamber, i.e. the Q of the chamber divided by the frequency of operation, and thereby also the coherence (or correlation) bandwidth. Preferably, the loading is not too strong, because then the resonances of the cavity modes disappear, and the chamber will no more have the desired function.
The reverberation chamber can always be improved with respect to both measurement accuracy and resemblance to practical environments. The uncertainty is at present good enough compared to alternative measurement techniques, but a more accurate chamber will allow measurements at a lower frequency or in a smaller chamber and at shorter time, which is attractive. The reverberation chamber represents an isotropic multipath environment with a uniform distribution of angles of arrival of the incoming waves over the complete surrounding space. This is a good reference environment for antennas and wireless terminals in multipath with fading (P.-S. Kildal and K. Rosengren, “Correlation and capacity of MIMO systems and mutual coupling, radiation efficiency and diversity gain of their antennas: Simulations and measurements in reverberation chamber”, IEEE Communications Magazine, vol. 42, no. 12, pp. 102-112, December 2004). Still, the time delay spread and coherence bandwidth need to be controlled in order to resemble different environments (X. Chen, P.-S. Kildal, C. Orlenius, J. Carlsson, “Channel sounding of loaded reverberation chamber for Over-the-Air testing of wireless devices—coherence bandwidth versus average mode bandwidth and delay spread”, IEEE Antennas and Wireless Propagation Letters, vol. 8, pp. 678-681, 2009). Unfortunately, such control by loading the chamber also affects the measurement uncertainty in a bad way.
The measurement uncertainty is generally described in terms of a standard deviation (STD) around the average, where the average is an estimate of the true value. Estimation of efficiency and related quantities like radiated power and receiver sensitivity is based on averaging over many samples (one for each different stirrer position). The STD is in such case inversely proportional to the square root of the number of independent samples, according to statistical theory. The number of independent samples is generally taken to be proportional to the number of excited modes in the reverberation chamber ([1] J. G. Kostas and B. Boverie, “Statistical model for a mode-stirred chamber,” IEEE trans. Electromagn. Compat., vol. 33, no. 4, pp. 366-370, November 1991), but by moving the antenna around in the chamber this value can be increased to the total number of waves, that is approximately eight times the number of excited modes (K. Rosengren, P.-S. Kildal, “Study of distributions of modes and plane waves in re-verberation chamber for the characterization of antennas in a multipath environment”, Microwave and Optical Technology Letters, Vol. 30, No 6, pp 386-391, September 2001). However, this is only true at low frequencies. When frequency increases the uncertainty does not improve accordingly, and is instead limited by a residual error that can be interpreted as direct coupling between the transmitting and receiving antennas or similar (P.-S. Kildal, S. Lai, and X. Chen, “Direct Coupling as a Residual Error Contribution During OTA Measurements of Wireless Devices in Reverberation Chamber”, IEEE AP-S International Symposium, Charleston, Jun. 1-5, 2009).
Similar measurement chambers are also disclosed in DE 198 12 923, WO 2010/026274 and WO 2005/003795. However, the therein-disclosed solutions are all subject to similar problems in respect of measurement accuracy and the like.
Thus, despite the improvements in measurement accuracy obtained by means of the reverberation chamber in recent years, there is still a need for improvements to enhance the measurement accuracy even further.